The present invention relates to switching power supply control circuitry and more particularly to power switch on time control and feedback loop stability.
Switching power supplies frequently employ a timing circuit to set the power switch on time. The timing circuit can use a capacitor charged by a current source from an initial voltage level when the switch is turned on to a final level when the switch is turned off. The on time of the switch is determined by the size of the capacitor, the magnitude of the charging current and the difference between the initial and final voltage levels.
In most control schemes the duration of the switch on time is varied by a feedback control circuit to compensate for changes in the input voltage and load current. In some applications, the range of switch on times is small, for example in a supply operating in continuous conduction mode from a telecommunication central office battery. In other applications, the range of on times can be very large, as can occur in a universal input off line supply operating in a discontinuous conduction mode.
Switch on time is usually controlled by varying the upper voltage threshold level. If the level is raised, the switch will remain on for a longer period, and as the upper threshold is lowered toward the initial threshold, the on time will decrease. On time is a linear function of the upper threshold voltage and is reduced to zero when the upper threshold is equal to the initial voltage level.
Difficulties occur when the required range of on times is very large. When short on times are required the upper threshold voltage falls very close to the initial voltage so small variations in voltage level due to noise cause a large proportional change in on time. This can cause jitter on the waveforms, oscillation, or limit cycle operation.
Output voltage regulation of a switching power supply is accomplished by comparing the output voltage of the power supply with a reference to generate a signal proportional to their difference. This signal is used to control the power switch on and off times to keep the output voltage at the desired value. Both the DC and AC gain of the feedback control loop must be controlled to get optimum DC regulation and transient response. The gain of the power train may be dependent on input voltage or load current, and the gain of the power switch on time controller may depend on the value of the the control signal. To keep loop gain constant over all operating conditions it is sometimes necessary to measure these parameters and add a network to algebraically modify the error signal based on their values.
It is an object of the present invention to provide a power switch on time control circuit that can operate over a wide range of on times with reduced sensitivity to noise.
It is another object of the present invention to provide a critically continuous boost converter with an output voltage feedback control circuit having a gain which is independent of operating point.